1. Field of the Invention
The present invention relates generally to method and apparatus for cleaning a liquid delivery system, e.g., of a type as used for generating a precursor vapor from a liquid precursor composition, for vapor transport to a deposition chamber for formation of a material film on a substrate.
2. Description of the Related Art
Chemical vapor deposition (CVD) is a highly desirable technique for depositing layers of thin film materials in the fabrication of semiconductor products.
In the CVD process, reactive precursor vapors are transported to a vapor deposition reactor, in which the vapor is contacted with a substrate at sufficiently elevated temperature to form a film thereon of deposited material from the precursor vapor. For many years, vapor delivery was accomplished by the use of bubblers in which inert gases were bubbled through volatile liquid reagents to effect saturation of the gas and transport of the resultant carrier gas/precursor vapor mixture to the reactor.
The application of new materials in semiconductor device manufacturing has dictated the corresponding use of new precursors for such materials. Many of these emerging precursors are non-volatile. For various reasons, including rate control, safety, reagent stability, and variety of available precursor materials, liquid delivery systems have replaced bubbler-based vapor transport systems for many materials and applications. In liquid delivery systems, a precursor liquid composition is metered and delivered to a vaporizer, with the resultant precursor vapor being flowed to the CVD process.
The continuing emergence of new chemistries poses novel and increasing challenges to the use of liquid delivery process systems. One example of such chemistries is the precursors available for CVD of copper thin films for interconnects in microelectronic device structures.
Copper is an important material in microelectronic device fabrication. Its high conductivity facilitates the formation of interconnects with greatly increased speed of response and smaller "landscape" requirements, relative to other metallization conductor materials such as aluminum. Various major semiconductor device manufacturers have targeted copper CVD as a major objective for development. The success of copper CVD hinges critically on the improvement of delivery and vaporization technology.
Copper precursors typically have low vapor pressure until they are heated within a few degrees of their decomposition temperatures. In order to prepare a concentrated reactive vapor from such precursors, it is necessary to heat these precursors to a temperature that is very close to the temperature at which decomposition occurs.
As a result, some decomposition unavoidably occurs. The extent of such decomposition may be minimized to some degree by the use of high precision process controllers for the liquid delivery system, but invariably there is some build-up of deposits, particles, clogs, byproducts, sludge, and/or other contamination which adversely affects vaporizer performance and drastically shortens the useful operating life of the liquid delivery system between maintenance events (when shutdown and cleaning of the system equipment is required).
Any significant shortening of the on-stream operating life of the liquid delivery system by reason of solids accumulation or other contamination therein is clearly undesirable. Further, some precursor compositions produce contamination that is very difficult, time-consuming and costly to remove.
The current state-of-the-art approach in liquid precursor vaporization using liquid delivery systems involves the use of high surface area vaporization elements, such as may be formed of sintered porous metal `frit`material, stacks of high aspect ratio disks, etc., onto which the precursor liquid is flowed. The vaporization element is heated to an appropriate temperature for high-rate volatilization ("flash vaporization") of the introduced liquid, and normally a means (e.g., a carrier gas flow stream, blower, eductor, and/or other structure producing desired flow/pressure drop characteristics) to sweep away the precursor vapors as they are generated.
For chemistries that are prone to partial decomposition within the vaporizer, such as copper, the heated areas of the vaporizer may be flushed with a solvent medium between deposition cycles, as more fully described in U.S. Pat. No. 5,362,328 issued Nov. 8, 1994 in the names of Robin A. Gardiner, et al. for "Apparatus and Method for Delivering Reagents in Vapor Form to a CVD Reactor, Incorporating a Cleaning Subsystem," the disclosure of which hereby is incorporated herein by reference in its entirety.
In such "solvent flush" approach, the solvent medium is selected to dissolve any residual liquid and/or solid residue from the surfaces of the vaporization element and the interior surfaces of the vaporizer. The solvent flush step typically is conducted once every several hours of run time, and the liquid delivery system is equipped with a vent or a reactor bypass circuit to transport the solvent waste from the solvent flush step to a site for recycle, solvent recovery, filtration removal of contaminants, or other disposition.
However, with sensitive chemistries such as copper, the flush step is required quite frequently--e.g., between each and every coating cycle. This can substantially lengthen the process cycle and generate excessive amounts of waste in the process system.